The present invention relates to optical multiplex transmission-type field instrumentation systems. More particularly, the invention relates to an optical multiplex transmission-type field instrumentation system in which data from a plurality of field devices, such as digital measuring units and field controllers for controlling operation terminals, is transmitted in a multiplex mode through optical fiber transmission paths and an optical distributor such as a star coupler to a master processor or a higher processing device on the side of a panel or centralized control room.
In general, in an instrumentation measurement system, a number of sensors or measuring units are installed "in the field", and measurement data from these sensors or measuring units is transmitted to a centralized control room located far from the field to thus monitor and control the particular process with which the control system is associated. Most conventional systems of this type are adversely affected by noise or line surges because they employ electrical signals. Furthermore, the conventional systems suffer from the difficulty that, when they are operated in an explosive atmosphere, it is necessary to provide suitable countermeasures. The above-described sensors or measuring units are, in general, of the analog type. Accordingly, they are adversely affected by external disturbances such as noise and temperature changes, and therefore their accuracy is low.
In order to overcome the above-described difficulties, the present applicant has proposed in Japanese Pat. Appln. 118414/81 a measured data optical multiplex transmission system in which, in order to transmit optical data through a two-way optical transmission path between N digital measuring units measuring physical data and a higher processing device or a master processor, an optical distributor is used to optically couple the single higher processing device and the N measuring units. The optical distributor branches in a ratio of 1:N and optically couples in a ratio of N:1 the optical data which is transmitted bidirectionally through the optical transmission path. Measured data is therein transmitted in a time-division multiplex mode between the measuring units and the higher processing device.
An object of this invention is to provide a field instrumentation system based upon the earlier-proposed system, but which is greatly rationalized and improved in reliability.